Conventionally, there has been known a technology regarding a power transmission apparatus included in a construction equipment vehicle such as a hydraulic excavator, wherein a load is driven based on motive power from a power source. For example, see the description disclosed by patent document 1.
The power transmission apparatus disclosed by patent document 1 includes an engine which serves as the power source and a hydraulic pump that is driven based on the motive power of the engine and discharges hydraulic oil.
When the hydraulic pump is driven based on the motive power of the engine, the hydraulic pump discharges the hydraulic oil. The hydraulic oil is supplied from the hydraulic pump to a working hydraulic actuator and a hydraulic motor, whereby the working hydraulic actuator and the hydraulic motor are driven. A working machine can be operated by the working hydraulic actuator to be driven. Also, a working vehicle can be travelled by the hydraulic motor to be driven.
With respect to the power transmission apparatus disclosed by patent document 1, the rated output of the engine is set corresponding to the absorbing horsepower of the hydraulic pump at the time of heavy load works, in order to prevent the occurrence of the engine stalling at the time of the heavy load works. That is, the rated output of the engine is set to a value larger than the maximum value of the absorbing horsepower of the hydraulic pump. Herein, by the heavy load works is meant a work during which the absorbing horsepower of the hydraulic pump is relatively high. As one example of the heavy load works, there are works such as “excavation loading work” in which the hydraulic excavator including the power transmission apparatus excavates the ground and loads a truck with earth and “travel earth removal work” in which a dozer thrusts the earth aside while the hydraulic excavator travels.
However, works performed by the working vehicle such as the hydraulic excavator including the power transmission apparatus include light load works, in addition to the heavy load works described above. Herein, by the light load works is meant a work during which the absorbing horsepower of the hydraulic pump is relatively low. As one example of the light load works, there are works such as “shoveling work” in which earth laid on the ground is shoveled up by the hydraulic excavator including the power transmission apparatus and “crane work” in which goods are suspended and transferred.
As is described above, when the maximum output of the engine is set corresponding to the absorbing horsepower of the hydraulic pump at the time of heavy load works, the settings of the maximum output of the engine are excessively increased at the time of light load works, and there is a disadvantage in that the deterioration of the fuel consumption of the engine, the increase of noise, and the reduction of heat balance performance are involved, and the reduction of energy efficiency is involved.